The instant invention deals in the art of security validation apparatus and particularly with an improvement therein. Heretofore, in validating currency or other securities, it has been known that certain designs or patterns within the currency or security may be correlated or masked against a reference to determine validity. However, with modern day reproduction apparatus it has been found that these primary tests of validation may be deceived by means of high resolution photo copies or the like. It has been further known, however, that photo copies are, for the most part, reproduced with an ink which is highly absorptive to infrared and/or visible light. However, some valid currencies, notes and other securities have certain areas thereon which are largely reflective to such light. While valid currencies contain these highly reflective areas, a photo copy of the same will be absorptive to the light in the correlated areas. Consequently, in the past a test has been proposed for checking the authenticity of a paper being passed as a valid security by sensing the infrared or visible light reflective or absorptive characteristics of certain areas of the paper.
Referring now to FIG. 1, there is shown a prior art teaching or apparatus utilized for the above-mentioned test. Since this test is generally incorporated in addition to a primary test of pattern recognition, the apparatus of FIG. 1 is often referred to as a secondary detection system. It can be seen that this system, designated generally by the numeral 10, includes a lamp 12 which emits a light preferably in the infrared range. A portion of the light emitted from the lamp 12 is passed to a solar cell 14 by means of a vane 16 appropriately positioned and angled. As will be appreciated later, the vane 16 is preferably of a reflective metallic nature. Other portions of light emitted from the lamp 12 strike the paper 20 which is to be tested for validity and are reflected therefrom toward a second solar cell 18. In general, the paper 20 will be so positioned above the lamp 12 that an area 22, being generally absorptive as to infrared and/or visible light, is in close juxtaposition thereto. It is the light reflected from the area 22 which is received by the solar cell 18. In consideration of the showing of FIG. 1, it should be particularly appreciated that the amount of light received by the solar cell 14 from the lamp 12 is directly dependent upon the positioning and angling of the vane 16.
With reference now to FIG. 2, a schematic diagram of the sensing circuit of the prior art may be seen as designated generally by the numeral 23. It should be noted that the solar cells 14, 18 are differentially connected to the amplifier 24; the amplifier having a feedback resistor 25 connected thereto. The output of the amplifier 24 is directly dependent upon the differential of current flow through the solar cells 14, 18. Of course, as is well known in the art, the amount of current flowing through a solar cell is directly proportional to the amount of light impinging thereon. Consequently, if both solar cells 14, 18 receive the same amount of light incident thereto, the output of the amplifier 24 will be null. As variations of light intensity incident to the solar cells 14, 18 change, positive or negative output voltage levels will be evidenced at the output of the amplifier 24, these voltage levels being indicative of the discrepancy between the amount of light incident to the solar cell 14 and that incident to the solar cell 18. If the solar cell 14 is designated as a reference solar cell and the vane 16 is physically adjusted, by positioning, bending and the like, such that the solar cell 14 receives the same amount of light as would be reflected to the solar cell 18 from the area 22 of a properly positioned valid paper 20, then the current produced by the reference cell 14 will be identical to that produced by the sensing cell 18 when the paper 20 is a valid piece of currency or the like. Of course, the reflective characteristics of the paper 20 depend upon the age and wear experienced by the paper. Further, the exact vertical positioning of the paper 20 with respect to the light source 12 and the sensing cell 18 is critical in determining the amount of light reflected from the area 22 to the cell 18. Consequently, due to the aging, wear and positioning considerations recited directly above, the vane 16 is generally adjusted to cause the output of the solar cell 14 to be at the mid point of a bandwidth of acceptable current output levels of the sensing cell 18. Thus, the outputs of the amplifier 24 indicative of an acceptable note, piece of currency or the like is characterized by this bandwidth; any output falling therewithin being indicative of an acceptable instrument.
It should be appreciated with reference to the apparatus of FIGS. 1 and 2 above, that the sensitivity of the system presented is dependent upon any movement of the filament within the lamp 12. Once the light vane 16 has been properly adjusted, the system is tuned only in so far as no further physical movements occur within the system. If, by repetitive thermal expansion and contraction of the filament of the lamp 12, or by jarring or the like, the filament should happen to move, it should be readily apparent that the system integrity would be greatly diminished and that retuning would be necessary. A further problem with this prior art teaching is that it is indeed extremely difficult to tune a system by toying with the positioning and angling of the vane 16. A further inherent drawback of the prior art teaching is that height variations of the paper 20 from the lamp 12 result in different validity readings because the amount of light reflected from the area 22 to the cell 18 is directly dependent upon such spacing. Yet a further inherent drawback with the prior art teaching is that the bandwidth of an acceptable paper is defined by fixed levels or fixed level outputs from the amplifier 24 rather than relative level outputs automatically compensating for aging of the lamp 12 or shifting of the lamp filament.